BROWN OUT: As Domonic Brown of the Phillies trots around the bases after blasting a three-run home run, Mets reliever Robert Carson reacts during a five-run fifth inning for Philadelphia in a 9-4 rout of the Amazin’s. Photo: Getty Images

Someday the Mets will have a good bullpen.

Until they do, depressing days like yesterday at Citi Field will happen. General manager Sandy Alderson has not been able to build a shut-down bullpen and that continues to be one of the Mets’ biggest downfalls.

The recurring nightmare hit the Mets hard yesterday as the Phillies beat up young lefty reliever Robert Carson, scoring five times in the fifth inning, thanks to two long home runs, on their way to a 9-4 victory.

Overall, this pen continues to be bull.

The Mets’ bullpen came into the day with a 5.02 ERA, tied for last in the majors, and got worse, rising to 5.28.

This was a day the Mets thought they had improved their bullpen by sending struggling Josh Edgin (9.64 ERA) to Double-A Binghamton to get his confidence back. One reason Edgin was sent to Binghamton instead of Triple-A Las Vegas was to remain close to home, because the Mets know they are going to need help again in this continuing cycle of despair. Carson’s ERA zoomed to 16.20.

Shaun Marcum started for the first time this season. This was essentially a rehab start for the right-hander. He managed to last four innings, giving up three runs on five hits. The game was turned over to the bullpen with the Mets trailing, 3-2.

One of the problems is the starters don’t go deep into games and that adds to the workload. One terrible inning can ruin the day. The Mets are trying to fix the mess.

“We’ve already brought [the bullpen issues] up, so that there is some positive pressure on the situation,’’ catcher John Buck told The Post. “Make a shutdown inning the priority. Hopefully we can improve upon it. That’s the first step. It’s the maturity of guys saying, ‘We’ve got to do better.’ ’’

The real shame of this is that closer Bobby Parnell has done a solid job and posts a 1.08 ERA.

The Mets are 5-0 in games started by Matt Harvey and 5-12 in all other games. Harvey pitches deep into games.

“Our starters have to get us deeper in the game,” manager Terry Collins said. “We’re using guys in the fifth and sixth innings that should be pitching the seventh and eighth.”

There are fatal lapses. On Thursday lefty reliever Scott Rice gave up two runs in the ninth in a 3-2 loss to the Dodgers.

Carson, the other lefty in the Mets’ pen, was making only his second appearance of the year.

The Mets are fragile in many ways. Collins said that is one reason Edgin was not sent to Las Vegas.

“The worst place you can send a pitcher who is struggling mentally is to Las Vegas, or to Colorado Springs, or to Albuquerque — one of those places where you make a good pitch and the guy hits a two-run double,’’ said Collins, who spent 13 years in the Pacific Coast League. “So I just said, ‘Go to Binghamton.’ ’’

The Phillies, despite all their problems, still own the Mets. They have won five straight at Citi Field and nine of the last 12. In those 12 games, they have scored 80 runs.

Domonic Brown crushed a three-run home run to right in the fifth off Carson and the next batter, John Mayberry Jr., homered to left. Carson has allowed three home runs in 3 1/3 innings this season.

He’s not facing minor league hitters anymore.

“If I miss my location at this level, that will happen,’’ Carson said of the home runs.

Frank Francisco, who was supposed to be the bullpen’s savior, continues to rehab in Port St. Lucie, coming back from an elbow cleanout, a relatively minor procedure. Francisco was signed to a $12 million deal before last season. He sports a 5.83 ERA as a Met.

After today, the Mets get a break, playing three in Miami against the miserable Marlins and then traveling to Atlanta to face the Braves, who have lost seven of nine.

This bullpen needs to turn it around immediately or this is going to be another long season for the Mets.